Starting with Unity 5.3, its installer lets you choose which platforms to support. This makes it easy to go with much smaller installs. With no extra platforms supported, my install is only 2 GB. That's much smaller than the default install of 5.2 which comes in around 7.5 GB.

How to fix "No module loaded"

If you don't select any extra platforms, you'll get the message like "No iOS Module Loaded". To solve this, go to the unity website and download installer for your version of Unity.

Run the Unity installer. Then select only the platforms you wish to install:

Once the Unity installer finishes, load Unity and you will be able to build on any of the platforms you installed.

How to save disk space in older versions of Unity

If you are still using 5.2 or older, trim down your install by deleting unnecessary files. These files are found in the PlaybackEngines folder.

I was able to reclaim 6 GB of diskspace and still deploy builds to Windows and Andriod:

If you delete a playbackengine, the only side effect is that you cannot build for that platform. Unity will display No module X loaded if you select the module in the Build Settings dialog:

Do you really need files to support for Samsung Smart TV or Blackberry? What about ios? The iossupport folder is particularly large, taking up 3.5 GB in Unity 5.2. Unless I'm mistaken, I believe Windows machines cannot deploy to ios.

The subfolders you'll want to delete can be found in Unity/Editor/Data/PlaybackEngines.